Laser isotope separation and gas purification by multiple photon absorption enhanced by heating

Abstract

The efficiencies of laser isotope separation processes based on multiple photon absorption from infrared laser pulses are enhanced thermally by raising the temperature of the gas upon which the laser beam is incident, thereby increasing the absorption and the isotopically selective excitation which enables isotopes in the gas to be separated. This same enhancement is applicable to processes for laser purification of gases which involve multiple photon absorption from infrared laser pulses.

We claim: 
     
       1. In a method of separating a desired isotope or impurity from a gaseous mixture containing the isotope or impurity wherein the mixture is irradiated with a laser which selectively excites the desired isotope or impurity by means of multiphoton absorption of the energy of said laser, the improvement which comprises the step of enhancing multiphoton absorption by heating the gaseous mixture, said isotope or impurity possessing spectral absorption features sufficiently distinct from the features of the remainder of the mixture to allow selective excitation when heated.   
     
     
       2. The invention as set forth in claim 1 wherein said thermal enhancing step is carried out while the laser energy is incident upon the gas. 
     
     
       3. The invention as set forth in claim 1 wherein said heating is carried out to heat the gas above room temperature. 
     
     
       4. The invention as set forth in claim 1 wherein said heating is carried out to heat the gas to about 500 K. 
     
     
       5. The invention as set forth in claim 1 wherein said laser energy is applied to said gas in the form of pulses. 
     
     
       6. The invention as set forth in claim 5 wherein said pulses have a fluence in the range of 5 mJ/cm 2  -1 J/cm 2 . 
     
     
       7. The invention as set forth in claim 6 wherein said pulses are short pulses less than 80 ns in width. 
     
     
       8. The invention as set forth in claim 1 wherein said gas consists of molecules selected from MoF 6 , SeF 6 , and SF 6 . 
     
     
       9. The method for separation of isotopes or impurities present in the molecules of a gas which have a spectral absorption shift larger than their spectral thermal shift and are selectively excitable when heated which comprises the steps of causing said isotopes or impurities to absorb multiple photons to selectively raise said isotopes or impurities to an excited energy state, heating said gas to enhance the multiple photon absorption by said isotopes or impurities while they are being caused to absorb multiple photons, and extracting said excited isotopes. 
     
     
       10. The invention as set forth in claim 9 wherein said causing step is carried out by irradiating said molecules with laser energy of wavelengths in the spectral absorption band of said isotopes or impurities. 
     
     
       11. The invention as set forth in claim 10 wherein said laser energy is in the form of pulses. 
     
     
       12. The invention as set forth in claim 11 where the fluence of said energy incident upon said gas is in the range 5.0 mJ/cm 2  to 1.0 J/cm 2 . 
     
     
       13. The invention as set forth in claim 9 wherein said heating step is carried out by heating said gas to raise its temperature from 199 K. to 600 K. 
     
     
       14. The invention as set forth in claim 9 wherein said gas consists of molecules selected from the molecules listed in MoF 6 , SeF 6 , and SF 6 .